The present invention relates to an improved shell and tube heat exchanger. In one aspect, the present invention pertains to a shell and tube heat exchanger useful in condensing a vapor stream containing some fraction of a non-condensible constituent. In another aspect, the present invention pertains to a shell and tube heat exchanger which is readily adaptable to a close-packed modular construction and which is able to endure the high thermally-induced stresses caused by changing temperature conditions.
The present invention is particularly well-suited for use the main condenser of an air separation plant double column, wherein the condensing nitrogen stream contains a small fraction of inert gases such as helium and neon which do not condense at the conditions prevailing in the main condenser. In this specific application, the main condenser provides two primary functions: it condenses the nitrogen separated in the lower column for its subsequent use as a reflux liquid in both the lower and upper columns, and it boils the liquid oxygen collected in the kettle of the upper column.
It is known in the art that the proper design of the main condenser is critical to achieve an energy efficient separation of air in a double column arrangement. An increase in the temperature difference between the shell and tube sides of the heat exchanger of 1.0.degree. K. represents about a 6 psi increase in the required head pressure for the entire air separation plant. Even more important, as readily recognized by one skilled in this technology, the design and operation of the main condenser is critical if one is to minimize the safety hazards normally associated with boiling liquid oxygen. As oxygen is evaporated to dryness, trace quantities of soluble hydrocarbons, which are normally present in the compressed feed air stream of the separation facility, are concentrated within the liquid. Eventually a combustible mixture may be formed which can explode violently. The design of the main condenser, therefore, must prevent this equilibrium boiling as well as the formation of localized pockets where boiling to dryness may occur.
A conventional main condenser in use today employs an open-ended vertical arrangement of boiling passages for the liquid oxygen. The boiling passages are partially submerged in a pool of liquid oxygen and the heat of vaporization is supplied by nitrogen condensing at a higher pressure (typically 110 psig) in heat transfer relationship with the boiling passages. The exchanger is designed such that the rate of vaporization within the passages is sufficient to entrain liquid with the rising vapor. With this open-ended design approach, the condenser can operate as a natural recirculation evaporator (thermo syphon reboiler), wherein the liquid entrained with the rising vapor is subsequently returned to the liquid pool and then to the boiling passages by gravity. As a result, not only is a constant supply of liquid provided to the boiling passages, but the plugging of individual passages and the concentration of hydrocarbons in the liquid are prevented by the flushing effect of the recirculating liquid.
Generally, plate type heat exchangers are employed for the main condenser in an air separation facility rather than shell and tube type exchangers because the plate type design is not plagued with the type of thermally induced stresses that one finds in a shell and tube exchanger, and the boiling passages are wide enough to eliminate the potential safety hazard involved with boiling liquid oxygen.
It is desirable to employ a shell and tube type heat exchanger in this application in order to increase the boiling heat transfer coefficient. However, with conventional shell and tube-type heat exchangers, excessive thermally imposed stresses may be created during transitional operating conditions, such as occur at start-up or shut-down. These stresses are caused by the temperature difference created between the tubes and the cylindrical shell during such transitional operating conditions. The tubes are thin walled members relative to the shell and therefore, their temperature will change much more rapidly in response to changing conditions than will the temperature of the shell. Accordingly, at any time when the temperature within the main condenser is changing, a temperature difference will be created between the tubes and the shell. Because the tubes are axially constrained by their rigid connection to the tube sheet, which in turn is rigidly connected to the cylindrical shell, the tubes are restrained from undergoing the thermal contraction or expansion coincident with their temperature. Instead, the tubes are restricted to the expansion or contraction of the shell, which because of its higher thermal inertia will be at a much lower rate than that of the tubes. As a result, depending upon whether the tubes are expanding or contracting, a large compressive or tensile load is applied to the tubes and the tube-type sheet joints. This load can be large enough to cause a failure of the joint or of an individual tube unless the temperature difference between the tube and the shell is adequately controlled. As a result, elaborate procedures and instrumentation are required for cooldown (start-up) and thawing (shut-down) of the main condenser to prevent a premature failure of the equipment.
Heat exchange prior art illustrates a possible solution to the above problem. In U.S. Pat. No. 2,254,070-Jacocks, for example, an expansion member is used as part of the cylindrical shell; while in U.S. Pat. No. 2,468,903-Villiger, an expansion member is used to join one of the spaced tube sheets to the cylindrical shell. Jacocks is particularly noteworthy in that it also illustrates the additional concept of employing a modular heat exchanger approach, wherein a single exchanger is fabricated from a number of individual heat exchanger components. This latter concept is especially important because it allows one to use small, commercially available expansion members in the fabrication of the individual modules rather than having to independently fabricate a large, non-commercially avilable expansion member which would obviously entail a significantly higher production expense.
One problem which has plagued main condensers used in air separation facilities is the accumulation of non-condensible constituents within the heat exchanger. When the vapor to be condensed contains a fraction which will require a significantly lower temperature for condensation, this fraction will build up on the condensing side of the heat exchanger and increase the temperature difference between the exchanging fluids. This buildup will increase until either the corresponding solubility of the non-condensibles in the condensed liquid allows their removal with the liquid at a rate equal to the rate at which the non-condensibles are introduced into the exchanger, or, in the limit, until the heat exchanger becomes vapor bound. In either case, the efficiency of the heat exchanger is drastically affected.
A problem associated with modular heat exchangers is the packing efficiency or the proximity with which the modules can be placed relative to each other. The packing efficiency of the modular assembly is a very important aspect of a main condenser design, since this determines the overall diameter of the main condenser needed to supply the necessary heat transfer area. The diameter of the main condenser is an important consideration for several reasons. First of all, transportation laws and regulations impose an upper limit on the diameter of equipment that can be shipped in interstate commerce. Secondly, in the most preferred configuration of an air separation facility, the main condenser is positioned between the stacked lower and upper columns. As a result, the diameter of the main condenser cannot be markedly disproportionate to their respective diameters. Thirdly, the surface area of the main condenser varies as a square of its diameter. Since heat leak into the condenser is proportional to its surface area, minimizing the diameter of the condenser is a key factor for minimizing heat leak.
Accordingly, it is an object of the present invention to provide an open-ended shell and tube heat exchanger which is capable of withstanding large thermal gradients between the shell and the heat exchange tubes.
It is another object of this invention to provide a heat exchanger which is capable of operating safely in a pool of liquid oxygen containing trace amounts of soluble hydrocarbons.
It is a further object of this invention to provide a heat exchanger which is capable of operating in an efficient manner when condensing a vapor stream containing a portion of non-condensible gas.
It is still another object of this invention to provide a modular heat exchanger capable of very high thermal performance while occupying a minimum of space.